1. Field of the Invention
The invention is in the field of data transmission systems and particularly in the field of symbol timing (clock) recovery techniques for data transmission systems.
2. Background Art
In conventional data transmission systems information may be transmitted in the form of modulated waveforms. In order to retrieve the transmitted information at a receiver it is necessary to demodulate the received transmission. The demodulation process requires that a carrier reference signal and a clock signal synchronized to the symbol timing of the received modulated information be derived. It is conventional to provide symbol timing recovery circuits at the receiver to recover symbol timing from the received signal and produce a clock synchronized to the symbol timing of the modulated information. A typical symbol timing recovery circuit for producing a clock synchronized to the symbol timing of the modulated waveform is disclosed in U.S. Pat. No. 4,419,759, which issued on Dec. 6, 1983 to John J. Poklemba,as part of circuitry for achieving concurrent synchronization of carrier phase and clock timing in double-sideband, suppressed carrier transmission systems.
The symbol timing recovery circuit of the referenced Poklemba patent is illustrated in FIG. 1 herein. This figure corresponds to the symbol timing recovery portion of the Poklemba patent FIG. 6. A full discussion of the FIG. 1 circuitry is found in the Poklemba patent. Two clock synchronization loops for symbol timing recovery are illustrated. These loops perform parallel, coherently aiding operations on the a and b channels. The operations performed by the channels are combined to produce a composite error signal, which typically has a better signal-to-noise ratio than either of the individual channels. A first clock synchronization loop is responsive to the baseband waveform, a.sub.i (t), derived from the input signal r(t) through the mixer 102. A second clock synchronization loop is responsive to the baseband waveform, b.sub.i (t), derived from the modulated input, r(t) through the mixer 104.
The first clock synchronization loop includes data filter 110 and delay 150 each receiving the baseband data waveform, a.sub.i (t). The output of the data filter 110 is applied to comparator 114 which is sampled by the recovered synchronized clock R to produce a replica, A, of the data waveform a.sub.i (t). During clock recovery, the output of the delay 150 is applied to the mixer 200. The mixer multiplies the baseband data signal with the time derivative of the analog bit stream a supplied from the differentiation circuit 202. The analog bit stream a is effectively the analog data estimate of the modulating waveform a.sub.i (t). The output from the mixer 200 is an error signal which is proportional to the difference between the output from VCO 118 and the incoming data rate. After summing the a and b error signals in adder 207, the resultant is applied to loop filter 208 which is connected to the VCO 118 to synchronize the VCO output to the incoming data rate.
The second clock synchronizing loop of FIG. 1 includes data filter 112, sampled comparator 116, data filter 122, delay 152, mixer 204 and differentiation circuit 206. The operation of the second clock synchronization loop is similar to the operation of the first clock synchronization loop. The baseband data waveform corresponding to the modulating waveform b.sub.i (t) is multiplied by the time derivative of the analog bit stream b, which is an analog data estimate of the waveform b.sub.i (t), in the mixer 204. The mixer output is applied to the loop filter 208 through the summing circuit 207, with the loop filter output being applied as a resultant error signal to the VCO 118.
The signal timing recovery circuits for clock synchronization illustrated in the above referenced Poklemba patent are implemented for operation at a fixed data rate. It is sometimes desirable to adapt a symbol timing recovery circuit to receive variable data rates. A method for variable rate symbol timing recovery is disclosed in U.S. Pat. No. 3,959,601 which issued on May 25, 1976 to Olevsky et al.
FIG. 2 is a simplified block diagram of the Olevsky variable rate clock signal recovery circuitry. Basically Olevsky provides a two stage clock recovery scheme. A first stage consists of a transition detector 12, mixer 14, bandpass filter 16, variable time delay 18, mixer 20, lowpass filter 22, and a comparator 42. A frequency synthesizer 24 is provided to translate a variable rate input signal to a common IF frequency. As explained more fully in the Olevsky patent, this first stage produces a coarse symbol rate clock estimate R.sub.s The second stage is a phase-lock loop (PLL) that tracks R.sub.s with a board pull in range voltage controlled oscillator (VCO) 28. The second stage is included so that the PLL tracking bandwidth may be made narrower than that of the bandpass filter, resulting in a better clock estimage, R.sub.s. Also, when no data transitions are present and the clock component of R.sub.s is effectively zero, an output clock will be available.
A serious disadvantage of this technique is that the pull range of the VCO must be relatively large requiring the use of LC or RC resonance determining elements in the oscillator. For example, the pull range is typically greater than an octave, as described in the Olevsky patent. As a result, the output clock phase jitter performance of the PLL will be severely limited by the phase noise of such a VCO. The reason for this is that a large closed loop bandwidth is required to track out the phase noise of the local VCO. The larger the bandwidth, the greater is the output clock jitter due to incoming noise.
Applying the Olevsky divide by N method to the PLL, in the fixed data rate clock recovery circuit illustrated in FIG. 1 to produce a variable data rate clock recovery circuit results in an arrangement as illustrated in FIG. 3.
Like elements in FIGS. 1 and 3 are given common reference numerals. With reference to FIG. 3, an input signal i(t), which is an analog baseband representation of an input data sequence, is applied to data filter 110 and delay 150 through an analog to digital converter 149. Of course, for variable data rate operation the filter 110 and the delay 150 must be scaled according to the selected symbol clock, R.sub.s. When the input signal is analog to digital converted, and digital filtering and delay are used, the scaling will be automatic.
After the baseband input signal passes through the filter 110 and is sampled with the recovered symbol clock, R.sub.s, a very good estimate of the transmitted data sequence is obtained. This data estimate, I(t), is digitally differentiated in a differentiation circuit 202 and multiplied in mixer 200 with the delayed incoming baseband signal, I(t+T), to produce a symbol timing error signal. This error signal is applied to the loop filter 208. The loop filter is coupled to the VCO 118' to apply the error signal to the VCO.
As explained hereinbefore, the variable rate clock recovery scheme that uses a VCO with a divide by N divider to reproduce the variable symbol rates must use a VCO with a broad pull range, with its inherent disadvantages which include relatively poor stability and increased output phase jitter. For progressively lower data rates, the bit error rate of the data transmission becomes more degraded due to excess recovered clock jitter. A narrow crystal VCO has a typical pull range five hundred times narrower than that of the LC tank VCO used in the circuit of the Olevsky patent. While the crystal VCO has increased stability and improved output phase jitter compared to the LC or RC VCO, it cannot be practically used in a variable rate clock recovery circuit using a divide by N circuit as illustrated in FIG. 3 for the following reason.
Use of the crystal VCO dictates that only integer divisible clock rates are possible. For example, if the crystal VCO frequency were 10 MHz in a divide by N scheme, only clock rates in the neighborhood of 10, 5, 2.5, 1.6 MHz, etc. could be obtained. This is so because the crystal cannot be pulled far enough to lock onto frequencies between those listed. Moreover, the first two clock rates of this example reveals that an octave pull range VCO is necessary for synchronization at any k-digit rate between 5 and 10 MHz.